Basement Conversions in London: What to Know Before You Dig

In London, the only way to add space without moving is often down. A basement conversion can add a lot of square footage, and value, to a house. But it’s slow, expensive and full of things you can’t see until you start digging.

This is what we’d want any client to know before they commit.

First: can you dig at all?

Some houses simply can’t go down. Mains services, tube tunnels or the structure of the house itself can rule it out. Councils also limit how big and deep a basement can be. The days of three-storey “iceberg” basements are largely over.

The quickest reality check is your council’s planning website. Look at what it has approved nearby in the last few years.

The four types of basement

1. Converting an existing cellar. The simplest, if you have the head height. Expect it to shrink once it’s waterproofed (known as tanking) and the floors and services go in.
2. Lowering or extending an existing cellar. The floor comes up, foundations are often redone and party walls underpinned. You’ll need a new staircase. Digging out and removing soil by hand is slow, so costs climb.
3. Digging a new basement under an existing house. By far the most expensive and complex. Expect it to take well over a year.
4. Adding a basement to a new build. If you’re building from scratch, this is by far the cheapest way to go down.

Every quote you get for the dig is for a habitable shell. Flooring, lighting, joinery, doors and skirting come on top, and they add a lot.

What it costs. As a rough guide, this is what builders are quoting for the dig and shell in 2026. In London, expect the top of these ranges or more: many builders add 30–50% for tight sites and access.

Type of basement	Rough cost per square metre
Converting an existing cellar	£850–£2,800
Lowering or extending an existing cellar	£3,000–£5,500
New basement under an existing house	£4,500–£7,500

Ground conditions, the water table and access to the site all move the number a lot. So treat these as a starting point, not a quote.

Permissions

Planning permission. Turning an existing cellar into living space can sometimes go ahead without it, under permitted development. Digging a new basement, adding a lightwell, or changing how the front of the house looks usually needs it. Rules vary by borough, and conservation areas are stricter. The Planning Portal is a good starting point.

Building regulations. Any new habitable basement needs building regulations approval, whether it’s a converted cellar or a new dig.

Listed buildings. If your house is listed, you’ll need consent for work inside as well as out. See our guide to renovating a listed building.

Party walls. If you share a wall with neighbours, the Party Wall Act means you must tell them formally before you start. Talk to them early. They may already know what’s under your street.

What’s under your house

Much of London sits on clay, but never assume. Rock, gravel, sand, chalk and high water tables all change the engineering and the cost. A structural engineer will dig trial holes and test the soil before anyone commits. Even then, some surprises only appear once the dig starts.

Check, too, where your land ends. Do you own the ground under the pavement at the front? It matters if you want to extend outwards.

Making it somewhere you want to be

A basement is only worth the money if the space is good. That’s where design earns its fee.

• Light. Lightwells and glass bring in daylight. Warm lighting that mimics daylight does the rest.
• Height. Every centimetre counts. Plan the ceilings and service runs before the dig, not after.
• Air. Good ventilation stops a basement feeling like a basement.
• Bedrooms. A basement bedroom needs a safe way out in a fire, usually an escape window. Without one you can’t sell or let it as a bedroom.

A basement conversion is one of the longest jobs in residential building. Our project timeline guide explains why, and our guide to what an interior designer costs covers fees.

Tell us about your project. The first conversation is free, and we can usually meet within a few days. It’s as much about us checking we’re the right studio for your project as you checking us.